Microcellular plastic foam refers to a polymer that has been specially foamed so as to create micro-pores or cells (also sometime referred to as bubbles). The common definition includes foams having an average cell size on the order of 10 microns in diameter, and typically ranging from about 0.1 to about 100 microns in diameter. In comparison, conventional plastic foams typically have an average cell diameter ranging from about 100 to 500 microns. Because the cells of microcellular plastic foams are so small, to the casual observer these specialty foams generally retain the appearance of a solid plastic.
Microcellular plastic foams can be used in many applications such as, for example, insulation, packaging, structures, and filters (D. Klempner and K. C. Fritsch, eds., Handbook of Polymeric Foams and Foam Technology, Hanser Publishers, Munich (1991)). Microcellular plastic foams have many unique characteristics. Specifically, they offer superior mechanical, electrical, and thermal properties at reduced material weights and costs.
The process of making microcellular plastic foams has been developed based on a thermodynamic instability causing cell nucleation (J. E. Martini, S M Thesis, Department of Mech. Eng., MIT, Cambridge, Mass. (1981)). First, a polymer is saturated with a volatile foaming agent at a high pressure. Then, by means of a rapid pressure drop, the solubility of foaming agent impregnated within the polymer is decreased, and the polymer becomes supersaturated. The system is heated to soften the polymer matrix and a large number of cells are nucleated. The foaming agent diffuses both outwards and into a large number of small cells. Stated somewhat differently, microcellular plastic foam may be produced by saturating a polymer with a gas or supercritical fluid and using a thermodynamic instability, typically a rapid pressure drop, to generate billions of cells per cubic centimeter (i.e., bubble density of greater than 108 cells per cubic centimeter) within the polymer matrix.
U.S. Pat. No. 4,473,665 to Martini-Vvedensky et al., is directed to a two-stage method for foaming thermoplastics. That patent describes a first stage wherein a polymer is placed in a pressure vessel for saturation with high-pressure gas. During a second stage, the polymer is heated to the Tg at a much-reduced pressure. The gas, previously forced into the polymer at high-pressure, foams the polymer when the polymer temperature reaches a temperature that sufficiently softens the plastic.
U.S. Pat. No. 5,684,055 to Kumar et al. discloses a method for the semi-continuous production of microcellular foam articles. In a preferred embodiment, a roll of polymeric sheet is interleaved with a gas channeling means (e.g., porous paper, gauze, mesh, woven and non-woven fabrics) to yield an interleaved cylindrical roll. The interleaved roll is exposed to a non-reacting gas at elevated pressure for a period of time sufficient to achieve a desired concentration of gas within the polymer. The saturated polymer sheet is then separated from the gas channeling means and bubble nucleation and growth is initiated by heating the polymeric sheet. After foaming, bubble nucleation and growth is “quenched” by rapidly cooling the foamed polymeric sheet. The '055 patent is entirely silent with respect to how a foamed polymeric sheet is to be subsequently shaped into an article of manufacture.
For years, customers and food vendors searched for a disposable coffee cup that was not too hot to hold, would keep the coffee warm, and did not contaminate the coffee or our planet. Even though they appreciated their insulating quality, consumers rejected PS (polystyrene) foam cups as too polluting and unhealthy. Major coffee vendors have used an additional costly insulating sleeve to satisfy their customers. Many food-packaging products call for the insulating qualities of foam without the hazards of PS foam.
Thermoforming is a conventional method for forming three dimensional shapes from flat polymer sheets. The process heat softens the flat polymer sheet and then vacuum or pressure forms the sheet onto a die with the required shape. Thermoforming produces general packaging as well as PS Foam Clamshells and solid plastic cups. When thermoforming is used to form a deep product such as a coffee cup, etc. from a flat plastic or foamed sheet, it is termed a deep draw.
As is understood by those of skill in the art, thermoforming in general refers to a set of related processes for producing shaped articles of thermoplastic. Included in thermoforming are the processes of vacuum forming, pressure assisted thermoforming, high definition thermoforming, drape forming, press forming and line bending.
While advances have been made in the field of thermoformed-foamed polymers, there remains a need for improved products and methods related to the manufacture of such products. The present invention fulfills these needs and provides for further related advantages.